The present invention relates to a high-concentration starch adhesive having a high concentration of starch and having a viscosity stable over a long period of time and a process for preparing it.
In the paper-converting industry, an adhesive prepared by dispersing starch in water, an alcohol or the like and gelatinizing it by heat, steam or the like is usually used. It is difficult to prepare the starch adhesive having a high concentration, because its viscosity becomes high at a low temperature. To obtain the high-concentration starch adhesive, the gelatinized solution must be kept warm or a modified starch or dextrin is necessary to be used as a carrier. The keeping of the starch warm requires the equipments thereof, and the use of the processed starch or dextrin pose problems such as high costs and lowering of the adhesive power.
A laminating adhesive having a relatively high concentration used in the production of corrugated boards is prepared by a Stein-hall method (hereinafter referred to as SH method) wherein a carrier part comprising a starch paste completely gelatinized by an alkali and heat is mixed with a main part which is an ungelatinized starch.
The carrier used for maintaining the main starch, preventing the gelation and increasing the fluidity is prepared by adding sodium hydroxide to raw starch, processed starch or high-amylose starch, the amount of sodium hydroxide being 10 to 16% by weight based on the starch, and dissolving the starch by heat at 50.degree. to 60.degree. C. However, a carrier concentration higher than 15% can not be obtained, because the viscosity is high even in the presence of sodium hydroxide. Further, although the stable paste can be obtained in the initial stage of the preparation, the main starch is gradually swollen with sodium hydroxide contained in the carrier to increase the viscosity. Therefore, the main starch concentration is limited. Under these circumstances, the maximum total starch concentration of the adhesives prepared by the SH method is 25%.
Such a low-concentration adhesive containing a high water content reduces the adhesion speed and retards the drying of the adhesive layer in the adhesion step. Thus, the material to be adhered, such as paper is swelled by the absorption of water, to cause problems such as warp. Another problem of the low-concentration adhesive is that a lot of energy is necessitated for the evaporation of water.
Various methods were proposed for stabilizing the viscosity of the adhesives prepared by the SH method and increasing the concentration thereof. For example, a method wherein raw starch is mixed with an acid-treated starch, oxidized starch or dextrin to form a carrier part [Japanese Patent Publication for Opposition Purpose (hereinafter referred to as `J.P. KOKOKU`) No.55-35076)], a method wherein an etherified starch or oxidized starch with hypochlorite is used for preparing the carrier part (J.P. KOKOKU Nos.54-1575 and 56-43268), and a method wherein starch is oxidized with perborate to form a low-viscosity carrier [Japanese Patent Unexamined Published Application (hereinafter referred to as `J.P. KOKAI`) No.56-129278]. In these methods, the viscosity of the carrier part is reduced by the oxidation or the like to increase the concentration of the adhesive. However, even when sodium hydroxide is used in these methods, a concentration higher than 30% cannot be obtained. In addition, the adhesive power is reduced by the oxidation of the carrier starch. In another method wherein a high-amylose starch is used as a carrier starch the increase of the relative amount of amylose in the high-amylose starch is limited in view of breeding and, in addition, when the relative amount of amylose is increased, the paste is seriously gelled at a low temperature. Under these circumstances, the lightly oxidized starch or low-viscosity high-amylose starch is frequently used as a high-concentration carrier. However, even in such a case, the addition of a strong alkali such as NaOH or KOH in a large amount is indispensable for inhibiting the gelation of the paste to keep the fluidity and increasing the main keeping capacity. When such an alkali is used, the ungelatinized main starch added to the carrier part is swollen with the alkali used in a large amount, to make the viscosity of the adhesive unstable. A method wherein a modified starch is formed under high pressure is disclosed in J.P. KOKAI No.51-151345. However, the product has a high viscosity even at a high temperature and it cannot be used as an adhesive at ambient temperature.